In the oil and gas industry seismic tools are deployed downhole and on the surface to provide operationally significant information about borehole and formation attributes adjacent the tools. Borehole seismic data can be utilized to determine subsurface stratigraphy and refine surface seismic data. However, the conditions in which such seismic tools are deployed can be extremely harsh.
More specifically, drilling operations are performed by rotating a drill bit under high normal pressure of 20,000 pounds or so to crush through rock formations. The variable lithology of earth formations and the high pressure and operational temperatures of 150 to 175° C. make the environment adjacent a drill bit and drill collar very rugged and subject to high pressure shocks as the drill bit crushes through formations forming a rugose borehole. Notwithstanding this harsh operating environment it is desirable to make downhole logging while drilling measurements including measurements with relatively delicate seismic equipment such as geophones and accelerometers. Similarly, it is common to use seismic sensors in other rough and harsh operating conditions. The harsh environments however make use of delicate sensors such as geophones and accelerometers problematic.
In seismic data monitoring or collection conventional geophones or accelerometers may be used which include a coil assembly sensitively suspended in a magnetic field produced by a magnet assembly. Vibrations of the earth induced by seismic sources, for example, at the surface of the earth, produce relative motion between the suspended coil assembly and the magnetic field. This motion induces an electrical signal which is proportional to the relative velocity between the coil assembly and the casing of the geophone. When shocks occur due to the normal operation of the seismic tool the centering springs of the geophones can be damaged to an extent that the seismic tool must be retrieved from the borehole to change the geophone.
In the past, a significant amount of borehole and formation data has been acquired by embedding sensors within a drill collar so that logging operations can be performed concurrently with drilling. Although it would be desirable to supplement the large amount of data already acquired by direct measurement of seismic waves with a geophone in the past such delicate instruments have been unable to survive the harsh environment for a practical length of time.
The limitations of conventional seismic sensor designs for operation in environments noted in the preceding are not intended to be exhaustive but rather are among many which may tend to reduce the effectiveness of previously known sensor mechanisms in field operation. The above should be sufficient, however, to demonstrate that sensor structures existing in the past will admit to worthwhile improvement for harsh shock applications.